The present invention relates to a thin-film electron emitter having an electrode-insulator-electrode or an electrode-semiconductor-insulator-electrode stacked structure for emitting electrons into a vacuum, a display device using the same, and an applied machine such as an electron-beam lithography apparatus.
A thin-film electron emitter according to the present invention is an electron emission element using hot electrons generated by applying a high electric field to an insulator. As a typical example, an MIM (Metal-Insulator-Metal) electron emitter constructed by a thin film having a three-layer structure of the top electrode-insulator-base electrode will be explained. This applies voltage between the top electrode and the base electrode to emit electrons from the surface of the top electrode. The MIM electron emitter is disclosed in, for example, Japanese Laid-Open Patent Publication No. Hei 7-65710.
FIG. 2 shows the operating principle of an MIM electron emitter which is a typical example of a thin-film electron emitter. A driving voltage 20 is applied between a top electrode 11 and a base electrode 13 so that the electric field in an insulator 12 is above 1 to 10 MV/cm. Electrons near the Fermi level in the base electrode 13 pass through the barrier by tunneling phenomena and are injected into the conduction band of the insulator 12 and the top, electrode 11 so as to become hot electrons. Part of these hot electrons is scattered by the interaction thereof with a solid in the insulator 12 and the top electrode 11 and loses energy. As a result, at time of reaching the interface between the top electrode 11 and a vacuum 10, there are hot electrons having various energies. Of these hot electrons, the hot electrons having an energy above the work function xcfx86 of the top electrode 11 are emitted into the vacuum 10. Other hot electrons are flowed into the top electrode 11. An electric current flowing from the base electrode 13 to the top electrode 11 is called a diode current Id, and an electric current emitted into the vacuum 10 is called an emission current Ie. An electron emission efficiency Ie/Id is about 1/103 to 1/105. Electron emission by this principle in, for example, an An-Al2O3-Al structure is observed. The electron emitter has excellent electron emitter properties in that even when the surface of the top electrode 11 is contaminated due to attachment of ambient gas to change the work function xcfx86, this will not greatly affect the electron emission characteristic, and is expected as a new electron emitter.
As described above, the electron emission efficiency Ie/Id of a thin-film electron emitter is typically low and about 1/103 to 1/105. In order to obtain a desired emission current Ie, the diode current Id must be increased. There is the problem that the capacity of a bus line feeding an electric current to the electron emitter must be high, and the high output electric current of a driving circuit is needed. In particular, when plural thin-film electron emitters are arranged in two dimensions for use, plural electron emitters are connected to one bus line, therefore, the high electric current is a significant problem. Further, in order to flow a large amount of the diode current, a higher electric field must be applied to the insulator. This can shorten the operating life of the thin-film electron emitter.
An object of the present invention is to increase the electron emission efficiency of a thin-film electron emitter. The reason why the electron emission efficiency is low is that, as described in connection with FIG. 2, hot electrons are scattered in the insulator and the top electrode and lose energy. The percentage of contribution of the insulator and the top electrode to scattering depends on various conditions of the thin-film electron emitter structuring material and the film thickness of the insulator and the top electrode. Although not generally evaluated, in any case, it is certain that the scattering in the top electrode considerably contributes to the electron emission efficiency. When using a top electrode material with a low occurrence probability of hot electron scattering, the electron emission efficiency is enhanced. We have found from various studies that the degree of hot electron scattering is associated with density of states of an electrode material. More specifically, as the density of states near the Fermi level of a material is low, the probability of hot electron scattering is decreased, and the electron emission efficiency of a thin-film electron emitter using the same is enhanced.
This will be explained as follows. The hot electron scattering in a solid is governed chiefly by electronxe2x80x94electron scattering. FIG. 3 is a diagram schematically showing the energy states of electrons in a metal before and after scattering. An electronic state of a hot electron before scattering is indicated by 1, and its energy is indicated by E1, interacting with an electron in a state 2.
The state below Fermi level EF is occupied by electrons. States 3 and 4 of two electrons after scattering can only be states above the EF. Providing energy references with respect to the Fermi level, the law of energy conservation gives:
E1+E2=E3+E4 greater than 0
In other words, the hot electron of the energy E1 can interact with only a valence electron in the range of 0 to xe2x88x92E1, and can only be a state in the range of 0 to E1 after scattering. The hot electron scattering probability is in almost proportion to the number of densities of states D(E) in this range.
In order to,actually measure a difference in electron scattering degrees between metal materials, we have made samples in which the top electrode 11 of an MIM electron emitter is structured by a double-layer film of Mxe2x80x94Au (M=Au, Pt, Ir, Mo, or W) to measure electron emission efficiencies. As shown in FIG. 4, the electron emission efficiencies are increased in the order of W, Mo, Ir, Pt and Au. FIG. 5 shows densities of states of these metal materials. The number of densities of states existing in the range of xe2x88x927 eV to +7 eV is decreased in the order of W, Mo, Ir, Pt and Au. In this way, we have found the above-mentioned association of the hot electron scattering degree with the density of states of the materials.
A top electrode material with a low occurrence probability of hot electron scattering is found to have a low density of states near the Fermi level. In other words, it is understood that a material having a wide-bandgap near the Fermi level is particularly desirable.
An example in which n+xe2x88x92Si frequently used as an electrode material in a semiconductor process is employed for the top electrode of a thin-film electron emitter is reported in Journal of Vacuum Science and Technologies B, Vol. 11, pp. 429 to 432. In this document, this structure cannot obtain a sufficient emission efficiency. This shows that the Si bandgap (1.1 eV) is not enough to reduce the electron scattering probability.
With the above-mentioned studies, we have found the following materials as materials optimal for the top electrode material of a thin-film electron emitter. In other words, they have a bandgap wider than that of Si. The top electrode which flows a diode current must have low resistance.
As such materials, there are particularly conductive oxides. Among these, a group of materials called a transparent conductive film have a bandgap above about 3 eV to prevent light absorption and are suitable for the top electrode of a thin-film electron emitter since the resistivity is about 1xc3x9710xe2x88x924 to 8xc3x9710xe2x88x924 xcexa9cm, the resistivity is below 10xe2x88x923 xcexa9cm, and the conductivity is high. More specifically, typical examples include tin oxide, Sb-doped tin oxide, Sn-doped indium oxide (ITO, Indium Tin Oxide film), zinc oxide, and Cd2SnO4.
Examples of other conductive oxides include non-doped indium oxide, F-doped indium oxide, CdO, TiO2, CdIn2O4, Cd2SnO2, Zn2SnO4, and Al-, Ga-, or In-doped zinc oxide.
The resistivity of these materials is about 100 times higher than that of a metal such as Au. The resistivity is low enough in the case that an electric current is fed to the electron emission region by a bus line, and the area of the electron emission region is about tens to hundreds of xcexcm square. A requirement for the electrode resistance is determined whether the voltage drop due to electric current passage is within a desired range or not. When the length of a shorter side of the top electrode is L, the required resistance value is changed by dependence of Lxe2x88x921 to Lxe2x88x922. When a metal is used for the top electrode, it is possible to realize a thin-film electron emitter having an electron emission region area of about 1 mm square. When the electron emission region area is about tens to hundreds of xcexcm square, 100-times resistivity can be allowed.
As other optimum materials, there are so-called wide-bandgap semiconductors such as GaN or SiC. These having a bandgap above 3 eV are enough, and can enhance the conductivity by impurity doping. These are suitable for the top electrode of a thin-film electron emitter.
As other transparent conductive films, there are conductive borides and conductive nitrides. Specifically, there is LaB6 as the conductive boride, and there are TiN, ZrN, and HfN as the conductive nitride. In the conductive boride and the conductive nitride, the density of states (DOS) in the range of about 3 eV immediately below the Fermi level is extremely low. Since the DOS in this energy range is not zero, they cannot be necessarily called a wide-bandgap material. Since the DOS is extremely low, the probability of hot electron scattering is low by the above-mentioned principle, so that they are suitable for the top electrode of a thin-film electron emitter. Further, these compounds have the advantage that since the work function of the surface is low and about 2.6 to 4 eV, electron emission from the surface easily occurs.
When desiring to decrease the resistivity depending on the application, a metal such as Au or Pt having a low density of states near the Fermi level may be stacked on the above-mentioned material having a bandgap wider than the Si bandgap. In this case, the electron emission efficiency is naturally lowered as compared with a material with no metal film. However, the emission efficiency is enhanced as compared with an Ir-Au stacked film which has been used for long life. This is included in the scope of the present invention that a wide-bandgap material is used. Since Au or Pt has a low density of states near the Fermi level, the probability of hot electron scattering is small. On the other hand, since the sublimation enthalpy is small, metal atoms are easily migrated into the insulator, so that the life of a thin-film electron emitter is shortened. In the structure of the present invention, a material having a bandgap wider than that of Si is contacted onto the insulator, but Au or Pt is not contacted. This problem will not arise.
As apparent from the above-mentioned description, the present invention is effective for all types of electron emitters in which electrons pass through the electrode, and are emitted outside. Needless to say, these are included in the scope of the present invention. As an example of such as an electron emitter, there is included an electron emitter of a base electrode (metal)xe2x80x94semiconductor (Si)xe2x80x94insulator (SiO2)xe2x80x94top electrode structure described in, for example, Japanese Journal of Applied Physics, Vol. 36, Part 2, No. 7B, pp. L939 to L941 (1997). Otherwise, there is included an electron emitter of a base electrode (semiconductor, Si)xe2x80x94porous (Si)xe2x80x94top electrode structure described in, for example, Japanese Journal of Applied Physics, Vol. 34, Part 2, No. 6A, pp. L705 to L707 (1995).
The thin-film electron emitter according to the present invention has a high electron emission efficiency, and can obtain a high emission current by a low diode current. In addition, the thin-film electron emitter can easily be of thin-film electron emitter arrangement substrate structure arranging thin-film electron emitters in two dimensions. Using this, it is possible to realize a thin-film electron emitter applied display apparatus having long life and high brightness, and a thin-film electron emitter applied machine such as a thin-film electron emitter applied electron-beam lithography apparatus.
For example, the thin-film electron emitter applied display apparatus can be constructed by joining a thin-film electron emitter substrate arranging thin-film electron emitters in two dimensions together with a face plate coated with phosphors via a gap, and by sealing the both, and by pumping out the inside to keep it vacuum.
In addition, the thin-film electron emitter applied electron-beam lithography apparatus can be provided with a thin-film electron emitter and an electron lens acting on an electron beam from the electron emitter. In this case, when using a thin-film electron emitter arrangement substrate arranging thin-film electron emitters in two dimensions, it is possible to obtain a thin-film electron emitter applied electron-beam lithography apparatus which permits simultaneous IC pattern transfer.
The thin-film electron emitter, the thin-film electron emitter applied display apparatus, and the thin-film electron emitter applied machine of the present invention solve the foregoing problems by the following structures.
(1) A thin-film electron emitter having a structure to stack a base electrode, an insulator, and a top electrode in that order which emits electrons from the surface of the top electrode into a vacuum when a voltage of a polarity in which the top electrode is a positive voltage to the base electrode is applied between the base electrode and the top electrode, including: the top electrode having a material with a bandgap wider than that of Si and electrical conductivity as a structuring material.
(2) The thin-film electron emitter according to the (1), wherein the material is GaN or SiC.
(3) The thin-film electron emitter according to the (1), wherein the bandgap :of the material is above 3 eV.
(4) The thin-film electron emitter according to the (3), wherein the resistivity of the material is below 10xe2x88x92xcexa9cm.
(5) A thin-film electron emitter having a structure to stack a base electrode, an insulator, and a top electrode in that order which emits electrons from the surface of the top electrode into a vacuum when a voltage of a polarity in which the top electrode is a positive voltage to the base electrode is applied between the base electrode and the top electrode, including: the top electrode having a conductive oxide as a structuring material.
(6) The thin-film electron emitter according to the (5), wherein the conductive oxide has, as a main component, at least one selected from a group consisting of a tin oxide, an indium oxide, and a zinc oxide, and the top electrode has a single-layer film of the conductive oxide or a multi-layer film thereof.
(7) The thin-film electron emitter according to the (6), wherein antimony is doped into at least part of the tin oxide, tin is doped into at least part of the indium oxide, and aluminum is doped into at least part of the zinc oxide.
(8) A thin-film electron emitter having a structure to stack a base electrode, an insulator, and a top electrode in that order which emits electrons from the surface of the top electrode into a vacuum when a voltage of a polarity in which the top electrode is a positive voltage to the base electrode is applied between the base electrode and the top electrode, including: the top electrode having a conductive boride as a structuring material.
(9) A thin-film electron emitter having a structure to stack a base electrode, an insulator, and a top electrode in that order which emits electrons from the surface of the top electrode into a vacuum when a voltage of a polarity in which the top electrode is a positive voltage to the base electrode is applied between the base electrode and the top electrode, including: the top electrode having a conductive nitride as a structuring material.
(10) A thin-film electron emitter having a structure to stack a base electrode, an insulator, and a top electrode in that order which emits electrons from the surface of the top electrode into a vacuum when a voltage of a polarity in which the top electrode is a positive voltage to the base electrode is applied between the base electrode and the top electrode, including: the top electrode having a stacked film comprising of a film of a material with a bandgap wider than that of Si and electrical conductivity and a metal film.
(11) A thin-film electron emitter having a structure to stack a base electrode, an insulator, and a top electrode in that order which emits electrons from the surface of the top electrode into a vacuum when a voltage of a polarity in which the top electrode is a positive voltage to the base electrode is applied between the base electrode and the top electrode, including: the top electrode having a stacked film comprising of a conductive oxide film and a metal film.
(12) A thin-film electron emitter having a structure to stack a base electrode, an insulator, and a top electrode in that order which emits electrons from the surface of the top electrode into a vacuum when a voltage of a polarity in which the top electrode is a positive voltage to the base electrode is applied between the base electrode and the top electrode, including: the top electrode having a stacked film comprising of a plurality of films selected from a group consisting of a conductive oxide film, a conductive boride film, a conductive nitride film, and metal film.
(13) The thin-film electron emitter according to any one of the (1) to (12), wherein a semiconductor layer is formed between the base electrode and the insulator.
(14) A thin-film electron emitter applied machine including: as an electron emitter, a thin-film electron emitter arrangement substrate arranging a plurality of the thin-film electron emitters according to any one of the (1) to (13).
(15) A thin-film electron emitter applied machine including: as an electron emitter, a thin-film electron emitter arrangement substrate arranging in two dimensions the thin-film electron emitters according to any one of the (1) to (13).
(16) A display apparatus having a thin-film electron emitter arrangement substrate arranging in two dimensions the thin-film electron emitters according to any one of the (1) to (13), a face plate which is coated with phosphors and is disposed opposite to the substrate, and driving circuits.
(17) The display apparatus according to the (16), including: a bus line for feeding an electric current to the top electrode, the bus line being formed on the top electrode on the opposite side to the base electrode.
(18) The display apparatus according to the (17), wherein the bus line is a plating film having the top electrode as a seed film.
(19) An electron-beam lithography apparatus including: the thin-film electron emitter according to any one of the (1) to (13); and an electron lens acting on an electron beam from the electron emitter.
(20) An electron-beam. lithography apparatus including: a thin-film electron emitter arrangement substrate arranging in two dimensions the thin-film electron emitters according to any one of the (1) to (13); and an electron lens acting on an electron beam from the electron emitter.